I write in both English and German - regularly. It is not a great deal to switch languages when I do. I am wondering which email client can switch dictionaries automatically. I certainly have never experienced that. Maybe you could name them.
If you want to combine all the dictionaries into one, that you can also do. It works fairly well, but lowers the accuracy of spell checking. I have found once that a misspelled English word exists in the German dictionary. 